In the past, a number of incremental shaft encoders have been proposed, and one example of these has been a simple illuminated slit and moving grid arrangement. As the grid moves, its lines alternately cover and uncover the slit, modulating the light from the illumination means falling on the photodetector. This system is applicable for only relatively crude measurements, and the fact that the light diverges after leaving the slit makes it necessary that the moving grid be close to the slit. The slit can be made narrower to help this, but this reduces the amount of light available and lowers signal level. As the resolution requirements increase and/or the size decreases, spacings become impractical and diffraction effects begin to appear which give spurious results.
A refinement of the foregoing type of arrangement involves the incorporation of a collimating lens. The lens serves to lessen the spread of the beam after it passes through the slit and allows smaller line spacing, and consequently enables small size and/or better resolution of the moving part. As size is reduced and/or better resolution required, the collimating requirements become more severe so that the collimator becomes more expensive and larger in size. Ultimately, diffraction effects prevent further improvements.
Perhaps the most common embodiment of a shaft encoder is similar in principle to the slit/rotating grip type, but has modifications which improve its performance. This type consists of a mask of alternate opaque and transparent lines illuminated with collimated light. Next to this is a moving part containing alternate dark and transparent lines. As this latter part moves, its lines alternately block and unblock the light passing through the transparent portions of the fixed mask. Once more, collimation requirements become more severe as size is reduced or resolution increased and ultimately diffraction effects prevent any further improvement.
It was to overcome the disadvantages of these prior art arrangements that the present invention was evolved.